Process

Pyrolysis

Heating something without letting it burn — which drives off everything volatile and leaves the carbon behind.

Pyrolysis is heating an organic material in the absence of oxygen. Nothing burns, because there is nothing to burn with; instead the molecules break apart under heat alone.

What comes off is a mixture of gas, condensable vapour and tar. What stays is carbon, in the shape of whatever went in — which is why a piece of charcoal still shows the grain of the wood and a carbon fibre still has the cross-section of the polymer filament it was made from.

The distinction from combustion is the whole process and it is the one people find counterintuitive: a charcoal kiln does burn a little, deliberately, and the heat from that small burn pyrolyses the rest. Control the air and you get charcoal; let it in and you get ash.

Processing

The variables are temperature, heating rate and residence time, and moving them shifts the yield between the three products.

Slow pyrolysis at 300 to 500 °C over hours or days maximises the solid: charcoal, biochar, and the coke made from coal in the same way.

Fast pyrolysis at around 500 °C in seconds maximises the condensable liquid, which is bio-oil, and is the route being pursued for turning waste plastic and biomass back into a chemical feedstock. It is worth setting against mechanical recycling directly: pyrolysis breaks the polymer back to feedstock and will accept mixed and contaminated streams that no sorter can separate, at a real energy cost and with a yield that is disputed; mechanical recycling keeps the polymer intact and cheap and needs it clean. They are not competitors so much as answers to different waste.

High-temperature carbonisation, above 1,000 °C, drives off essentially everything but carbon. Carbon fibre is made this way, and the higher the final temperature the more ordered the carbon and the stiffer the fibre.

Historically the vapour was the point rather than the residue. Wood distillation supplied methanol — which is where 'wood alcohol' comes from — acetic acid and acetone before any of them could be made from petroleum, and coal gas lit European cities for a century as the by-product of making coke.

History

It is one of the oldest controlled chemical processes there is. Birch bark tar, made by pyrolysing bark in a sealed pit, was hafting stone tools in Europe at least 200,000 years ago — a manufactured adhesive older than our species, and one that requires excluding air from a fire on purpose.

Charcoal burning ran continuously from then until coke displaced it in metallurgy, and the collier's craft was entirely about airflow.

Its modern significance has inverted twice. Coke-making made it industrial. Petroleum made the by-product chemicals obsolete. And the search for something to do with waste plastic and for a way to store carbon in soil has made it interesting again, which is a reasonable summary of the last two centuries of a great many processes.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

produces

Charcoal — material · the kiln burns a small fraction of the wood to supply the heat that pyrolyses the rest
Carbon fibre — material · polyacrylonitrile filament, first oxidised in air at 200 to 300 °C so it will not melt, then carbonised above 1,000 °C — and the final temperature sets the stiffness, because it sets how ordered the carbon is

takes as input

Wood — material · and what comes off was the point for most of history: methanol, acetic acid and acetone all came from wood distillation before petroleum
Coal — rock · to make coke, and the coal gas that came off it lit European cities for a century as the by-product

Sources

Material World
Our own writing
Wikidata
Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)

Where it comes from, and what it becomes

Follow Pyrolysis back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

Pyrolysis → takes as input (to make coke, and the coal gas that came off it lit European cities for a century as the by-product) → Coal → is sourced from (every coal seam in the world was a peat bog. Bury it, compress it, heat it, and it passes through lignite and bituminous coal to anthracite, losing water and volatiles at each step) → Peat → is sourced from (waterlogging is the whole mechanism: a bog is anoxic below a few centimetres, so the organisms that would decompose the plant matter cannot work, and it accumulates instead) → Water → is sourced from (by desalination, where energy is cheap and coastline available — the only route that adds fresh water rather than moving it) → Seawater
Pyrolysis → takes as input (and what comes off was the point for most of history: methanol, acetic acid and acetone all came from wood distillation before petroleum) → Wood

Downstream — what it becomes

Pyrolysis → produces (the kiln burns a small fraction of the wood to supply the heat that pyrolyses the rest) → Charcoal → is an input to (as both the fuel and the reducing agent, and it smelted every metal humanity had until coke — which is why ironworks were built in woods rather than at the ore) → Smelting → produces (as blister copper, refined electrolytically afterwards) → Copper → is a component of → Bronze → is associated with (the alloy the period is named for) → Bronze Age complete chain
Pyrolysis → produces (polyacrylonitrile filament, first oxidised in air at 200 to 300 °C so it will not melt, then carbonised above 1,000 °C — and the final temperature sets the stiffness, because it sets how ordered the carbon is) → Carbon fibre → is used in (in the spar caps of the longest blades, where stiffness stops the tip striking the tower) → Wind turbine blade → is used in (and it is the part of a turbine with no established end-of-life route, where the tower and foundation are steel and concrete) → Energy generation
Pyrolysis → produces (polyacrylonitrile filament, first oxidised in air at 200 to 300 °C so it will not melt, then carbonised above 1,000 °C — and the final temperature sets the stiffness, because it sets how ordered the carbon is) → Carbon fibre → is a component of (and quoting the filament's strength for the laminate overstates it by a factor of several, which is the commonest error made about the material) → Carbon fibre composite → is used as (aircraft primary structure since the 1990s, where stiffness per unit mass is what is being bought) → Structural engineering
Pyrolysis → produces (polyacrylonitrile filament, first oxidised in air at 200 to 300 °C so it will not melt, then carbonised above 1,000 °C — and the final temperature sets the stiffness, because it sets how ordered the carbon is) → Carbon fibre → is used as (where stiffness per unit mass justifies the cost — aircraft structure, pressure vessels, and the reinforcement of existing concrete) → Structural engineering
Pyrolysis → produces (polyacrylonitrile filament, first oxidised in air at 200 to 300 °C so it will not melt, then carbonised above 1,000 °C — and the final temperature sets the stiffness, because it sets how ordered the carbon is) → Carbon fibre → is used as (as the fibre in a composite rather than as fabric in its own right, though it is woven first) → Textiles
Pyrolysis → produces (polyacrylonitrile filament, first oxidised in air at 200 to 300 °C so it will not melt, then carbonised above 1,000 °C — and the final temperature sets the stiffness, because it sets how ordered the carbon is) → Carbon fibre → is used in (primary structure since the 1990s — a modern wide-body wing and fuselage are more composite than metal) → Aerospace manufacture

These are the most distinct paths onward. Pyrolysis ends up in others besides.